In multiple color printing, multiple step printing processes are employed in which different color components of multiple color images are separately imprinted onto the printing stock, usually paper, in a sequence of impressions with a series of separate plates or mats mounted onto separate cylinders of a press. Each of the mats imprints with an ink of a different color. Typically, four colors are employed. In such processes, it is important that the various plates be mounted on the cylinders in precise registration with each other so that the colors precisely align for the production of a high quality multicolor image.
In printing processes that use flexographic printing plates, or flexible photopolymer mats, the plates are usually secured to the cylinders with an adhesive such as a double backed tape, which holds the plates in position during printing. In mounting such plates, care must be taken to register the plates precisely in the same relative position on the respective cylinders. This registration process is often assisted with a mounting apparatus sometimes referred to as a registration system. These registration systems frequently employ pins that receive the mats by engaging a pair of holes made in the mats for registration purposes. The pins are locked into place on a mounting table, and the mats are transferred to the respective cylinders, one at a time, in what is intended to be the same relative position with respect to the respective cylinder. Inevitably, however, some error exists which will cause a slight misregistration of the colors on the printed work which detracts from or limits its quality.
Registration systems of the prior art have operated to engage the mats and transfer them to the print cylinders so as to register one edge of the plate along a line on the surface of the cylinder that is as close to being parallel to the axis of the cylinder, and as consistently located on the surface of the cylinder, as is possible with the equipment.
In such prior art systems, when the edge at one end of the mat is brought into contact with the cylinder surface, it is usually fastened to the surface first at this edge, and then progressively secured across its back surface to edge at its opposite end. In such a process, any error that exists in the registration of the first edge of the mat progressively increases around the cylinder with the distance from the edge of the mat that was first secured.
In one prior art system, it has been proposed to register the mats, begin securing the mats, from their centers. Such system employs pins mountable in holes predrilled into the cylinders to register with holes in the mats. This system has proved impractical because (1) it requires precise location of the holes in the mats to some predetermined positions or spacing, (2) it requires the drilling of multiple sets of holes in the cylinders to accommodate mats of different sizes, and (3) restricts the positioning of the holes in the mats to those of predetermined dimensions and spacing. These disadvantages make it most impractical to mount more than one mat at a time to a cylinder.
The systems of the prior art have been prone to error in the mounting of the plates on the cylinders, have required practice and skill in mounting the plates, and often consume excessive time and repetitive attempts to precisely mount the plates.
The prior art has not provided a flexographic mat registration system and method that is more precise and more practical than those set forth above. Accordingly, there is a need for a more precise, easier to use, and more flexible plate registration method and apparatus.